Talmud for Bava Batra 9:6
שְׁכִיב מְרַע שֶׁכָּתַב כָּל נְכָסָיו לַאֲחֵרִים וְשִׁיֵּר קַרְקַע כָּל שֶׁהוּא, מַתְּנָתוֹ קַיֶּמֶת. לֹא שִׁיֵּר קַרְקַע כָּל שֶׁהוּא, אֵין מַתְּנָתוֹ קַיֶּמֶת. לֹא כָתַב בָּהּ שְׁכִיב מְרַע, הוּא אוֹמֵר שְׁכִיב מְרַע הָיָה וְהֵן אוֹמְרִים בָּרִיא הָיָה, צָרִיךְ לְהָבִיא רְאָיָה שֶׁהָיָה שְׁכִיב מְרַע, דִּבְרֵי רַבִּי מֵאִיר. וַחֲכָמִים אוֹמְרִים, הַמּוֹצִיא מֵחֲבֵרוֹ עָלָיו הָרְאָיָה:
If a sh'chiv mera (one on his deathbed) wrote over all of his property to others, leaving over some land for himself, his gift stands. If he did not leave over any land, his gift does not stand. [The same applies if he left over movables. And if he rose (from his sickbed), he cannot retract. This, if he (the recipient) acquired that gift from his hand. For a limited gift of a sh'chiv mera requires a kinyan (an act of acquisition), whether or not he recovers. And if the recipient did not acquire it from his hand, he does not acquire it even if he died. This, only when he wishes to confer the gift while alive; but if he instructed that something be given to someone after his death, a kinyan is not required, even if he reserved something for himself. And a sh'chiv mera who wished to confer a gift while alive, if he reserved nothing for himself, and he recovered, he may retract, even if it was acquired from his hand. And when he so instructs because of (what he thinks is his) impending death (as when we hear him say: "Woe unto me, for I am going to die!"), he may always retract, whether or not he reserved something for himself, and even if it were acquired from his hand. And if he died, the other acquires the gift, even without a kinyan.] If he did not write him (in the deed of gift) "sh'chiv mera" [i.e., "when he was sick in bed," or an expression connoting health, e.g., "when he was walking on his feet in the marketplace," and he left over nothing] — If he says: "I was a sh'chiv mera" [and I retract], and the other says: "You were well" [and you cannot retract], he must bring proof that he was a sh'chiv mera. [For we follow the present state. Since he is well now, he must bring proof that he was a sh'chiv mera at the time of the gift.] These are the words of R. Meir. And the sages say: [We do not follow the present state, but we say that] the burden of proof is upon him who would exact (payment) from his neighbor. [And the would-be recipient, who wishes to exact (the gift) from the giver, who now holds it — it devolves upon him to bring proof and witnesses that he was well at that time. The halachah is in accordance with the sages.]